These comparative studies are aimed at deciphering which neurons are related to vision and how neural structures evolved in relation to the evolution of given functions. The long term goals are to find and characterize the structure of neuronal pools related to various visual functions. Although the vertebrate visual system is one of the most popular research areas, we still lack significant pieces of information about the range of variation in structural organization. This information could now be available with the use of modern neuroanatomical techniques. Our broad objectives are to determine the central visual pathways in a broad spectrum of vertebrate species ranging from elasmobranchs to primates and to characterize the cytoarchitectural and ultrastructural features of the pertinent cell groups. Over the past few years, we have, using modifications of the Nauta method for degenerating axoplasm, identified several new pathways from the retina, optic tectum, "visual thalamus," and "visual telencephalon" in different vertebrates. This preliminary work has revealed suggestive evolutionary trends as well as many new facts that require further study. There is still no information available about the precise origins or terminations of many visual pathways. The information sought is relevant to understanding human visual mechanisms because our studies will extend present knowledge about which cell aggregates are involved in vision and what features vary from species to species.